The topic of “Further Enhancements to LTE Time Division Duplex (TDD) for Downlink-Uplink (DL-UL) Interference Management and Traffic Adaptation” has been agreed as a study item in 3GPP release 11 and a work item in 3GPP release 12. Performance evaluation of various deployment scenarios has been conducted by both 3GPP RAN 1 and RAN 4 working groups. It has been shown that an average cell throughput can be improved to a large extend by allowing dynamic reconfigurations in Long Term Evolution (LTE) time division duplex (TDD) systems.
The TDD scheme would offer flexible deployments without requiring a multiple spectrum resources. Currently, the LTE TDD would allow for asymmetric downlink-uplink (DL-UL) allocations by providing seven different semi-statically configured DL-UL configurations as illustrated in FIG. 1A, and these allocations can provide between 40% and 90% DL subframes. To be more specific, the seven different semi-statically configured DL-UL configurations are indexed in the left most column of FIG. 1A and are numbered between 0˜6. In the present disclosure, a DL-UL configuration is also referred to as a TDD configuration or a TDD DL-UL configuration. For each TDD configuration, subframes of a radio frame would be configured as a downlink subframe, as an uplink subframe, or as a special subframe, and the top row of FIG. 1A shows the index of the subframe numbers. Therefore, in order to configure a radio frame to have a certain number of downlink and uplink slots, an evolved Node B (eNB) would transmit one of the UL-DL configurations in system information (SI).
For example, if heavy downlink traffic has been experienced by the network, the eNB could decide upon the TDD configuration 5 which would be transmitted to UEs and would provide 8 downlink slots and 1 uplink slot per radio frame. However, if heavy downlink traffic has all in a sudden been changed to heavy uplink traffic, the eNB may not change the TDD configuration instantly but has to convey the change by modifying the system information, and the modification of the system information for a legacy UE could only occur at a modification boundary. This would mean that the re-configuration of the TDD configuration via the SI change would be semi-static rather than dynamic and may not match the instantaneous traffic situation.
In comparison to the system information change procedure, known dynamic re-configuration techniques would require a much shorter period for TDD reconfiguration. Evaluation in the corresponding study item reveal significant performance benefits by allowing TDD DL-UL reconfiguration based on traffic adaptation in small cells as mentioned in “Further enhancements to LTE Time Division Duplex (TDD) for Downlink-Uplink (DL-UL) interference management and traffic adaptation,” 3GPP TR 38.828, V11.0.0, 2012-06, which is incorporated by reference for definition purposes. Also, it was shown that a dynamic signaling mechanism would outperform the mechanism that uses the system information change procedure.
Also for definition purposes, the TDD frame structure, DL-UL configurations, and the UL-HARQ timing relations would be defined according to “Physical Channels and Modulation,” 3GPP TS 36.211, V11.0.0, 2012-09, “Physical Layer Procedures,” 3GPP TS.213, V11.0.0, 2012-09, and “Medium Access Control (MAC) protocol specification,” 3GPP TS 36.321, which are both incorporated by reference for definition purposes.
However, using dynamic techniques to re-configure a TDD configuration would cause legacy UEs without a dynamic re-configuration capability and new UEs possessing such capability to have different understandings of the TDD DL-UL configuration, since legacy UEs must follow the system information change procedure while new UE would be able to re-configure the TDD DL-UL configuration via dynamic signaling mechanisms such as physical layer signaling, medium access control (MAC) signaling, or radio resource control (RRC) signaling. This could potentially lead to a variety of problems including problems caused by UE measurements as well as Hybrid Automatic Repeat Request (HARQ) operations.
HARQ is referred to as a transmission technique widely used in modern wireless communication systems. HARQ operates by re-transmitting an identical copy of the original transmission or another redundancy version upon transmission error. The receiver then combines the previously corrupted transmissions with the retransmitted one. In LTE TDD systems, the timing relation between the feedback information indicating a transmission error and corresponding retransmission are separately and differently defined for each of the 7 configurations due to the different allocation of the DL-UL subframes. However, sudden changes of TDD configuration could cause interferences of the HARQ operation between legacy UEs and new UEs having the dynamic re-configuration capability.
Also, dynamic re-configurations of the TDD configuration would not only cause problems between legacy UEs and new UEs having the dynamic re-configuration capability but also might cause interference among new UEs since new UEs may have different HARQ reference timings for both uplinks and downlinks depending on the TDD configuration which is dynamically selected. Furthermore, dynamic re-configurations of the TDD configuration would also cause problems in the soft buffer management process of new UEs.
More specifically, using dynamic techniques to re-configure the TDD configuration would affect the soft buffer management of a UE when the UE undergoes a HARQ process. The software buffer management during a DL HARQ process could be briefly described as that for each subframe in which a downlink transmission takes place between a UE and a base station, the UE would receive payloads into transport blocks and associated DL HARQ information from the base station. The UE would then either store the payload in the buffer or combines with the payload previously stored in the buffer according to the Log-Likelihood radio (LLR) information based on whether the downlink transmission is a new transmission or an old transmission. The UE would then respond with an ACK or NACK based on decoding result of the received payload stored in the buffer. For detailed discussion related to the soft buffer setting, please refer to the citations provided above. Also the UL HARQ would function in a similar manner.
However, dynamic alterations of the TDD configuration would potentially lead to instability of the soft buffer caused by sudden changes of the TDD configuration. FIG. 1B illustrates a maximum number of DL-HARQ processes (MDL-HARQ) for TDD in a typical LTE communication system. For the case of a frequency division duplex (FDD) system, the maximum of downlink HARQ processes is currently defined to be 8 per serving cell. For the case of a TDD system, it could be observed that the maximum number of HARQ processes per cell is not constant but would vary according to the current TDD UL/DL configuration set in the SI of a serving base station. For the case of downlink, the maximum number of DL-HARQ processes would affect how the soft buffer setting is configured by the UE. This would mean that when the TDD configuration is being dynamically altered, the soft buffer setting would also need to be dynamically re-configured. Thus, sudden alterations of a soft buffer setting before a UE would have time to react would result in losses of data previously stored in the soft buffer.
In addition, an important part of the support for downlink channel-dependent scheduling is channel-state report. The channel-state report is provided by the UE to the eNB, and eNB could make scheduling decisions based on the channel-state report. One type of the channel-state report is aperiodic channel-state report. Aperiodic channel-state reports are delivered when explicitly requested by the network by means of the channel-state-request flag included in uplink scheduling grants. Using dynamic techniques to re-configure the TDD configuration would cause timing mismatch in aperiodic channel-state reports.
As legacy UEs (before release 12) are not compatible with the technique of dynamic TDD DL-UL reconfiguration according to the challenges mentioned above, a new design could be required in order to avoid possible conflicts between legacy UEs and new UEs (release 12 and beyond). Also the new design would need to address possible conflicts among new UEs due to HARQ timing mismatches and problems related to soft buffer management.